add toml to shared-utils (#2198)

Author: Jason Yu

packages/@vuepress/shared-utils/package.json

@@ -38,6 +38,7 @@
     "gray-matter": "^4.0.1",
     "hash-sum": "^1.0.2",
     "semver": "^6.0.0",
+    "toml": "^3.0.0",
     "upath": "^1.1.0"
   },
   "devDependencies": {